[WIP] Adding time-varying covariates to AAF 

Adding NotImplementedExceptions to things that aren't working yet. No tests yet. 

To do:

 - [x] Tests for static and time-varying covariates
 - [x] Implement `predict` methods. 
 - [x] Documentation for new api
 - [x] Bump release to 0.3.0 (weeee!)
 - [ ] accept @manygrams PR for changing api to `event_occured`